1. Field of the Invention
The present invention relates to a display apparatus using a semiconductor device (typically a transistor) as a fundamental component, and in particular to a technical field related to display devices such as an electroluminescence display apparatus, a liquid crystal display apparatus, and a field-emission display apparatus and a technical field related to an electronic equipment in which the display apparatus is installed in an image display part.
2. Description of Related Art
In recent years, developments of liquid crystal display apparatus and electroluminescence display apparatus, in which transistors such as thin film transistors and MOS transistors are integrated on substrates, have been advanced. Each of them is characterized in that transistors are built on a glass substrate by means a thin-film forming technology and these transistors are placed on the respective pixels being arranged in matrix to provide the display apparatus with the function of displaying an image.
Referring to FIG. 11, there is illustrated the configuration of a typical active-matrix electroluminescence display apparatus. In FIG. 11, the reference numeral 1101 denotes a substrate on which thin film transistors 1102 are formed. Each thin film transistor 1102 is connected to a pixel electrode 1103 that acts as an anode of a light-emitting device . In addition, an insulating film 1104 is formed on the pixel electrode 1103. The insulating film 1104 has an opening at a position corresponding to the pixel electrode 1103. Furthermore, a metal film 1106 functioned as a cathode of each of a luminous body 1105 and a light-emitting device is placed over these structural components. Consequently, the electroluminescence display apparatus is configured as described above, so that an image display can be attained with light emission by a current injection to the luminous body 1105.
In this case, such an electroluminescence display apparatus may be fabricated in the usual process up to the formation of the pixel electrode 1103 and the insulating film 1104. In the case of using an organic compound as the luminous body 1105, alternatively, a film-forming method to be used may be a deposition method, an application method, an ink-jet method, or a printing method. Moreover, the formation of the metal film 1106 on these structural components is performed using a deposition method or a spattering method because the heat resistance of the luminous body 1105 is as low as 100° C. or less.
The present inventors found that an abnormal threshold voltage could be observed on the thin film transistor in the electroluminescence display apparatus configured as shown in FIG. 11, which was fabricated in the process of research and development. As a result of investigating the causes of such a phenomenon, the present inventors found that there was a large shift of threshold voltages (VG) before and after the formation of a metal film to be provided as a cathode by means of an electron-beam deposition method. The results are shown in FIGS. 12A and 12B. The data shown in FIGS. 12A and 12B are represented by characteristic curves of the drain voltage and gate voltage characteristics (hereinafter, referred to as ID-VG characteristics) of the thin film transistors before and after the formation of the metal film to be provided as a cathode. As is evident from FIGS. 12A and 12B, the threshold voltages of the thin film transistors after the formation of the cathode were shifted about 4 volts to the minus side, compared with the threshold voltages before the formation. Moreover, it was also confirmed that the value S (subthreshold coefficient) indicated an increase (i.e., deterioration) in the steepness of switching characteristics.
It could be considered that the threshold voltage and the value S were substantially changed as a result of causing damage on the thin film transistor at the time of forming the cathode.
The causes of abnormal characteristics of the thin film transistor may include malfunctions of the thin film transistor by damages caused by radiation and a deterioration of the thin film transistor by the generation of electrons or interface state density. The malfunction of the thin film transistor by damages caused by radiation is well known in persons skilled in the art. Generally, the malfunctions may be classified three types: the generation of positive charges caused in the oxide film by irradiation with of a radioactive ray (e.g., gamma ray, neutron, or X ray); the generation of an interface state density of the Si—SiO2 interface; and the generation of neutral electron trap in the oxide film. For malfunctions of transistors caused by radiation damages, the details are described in the publication entitled “Silicon thermal oxide film and Its Interface” (Kenji Taniguchi et al. Ed., pages 167 to 182, published on Jul. 31, 1991 from Realize Co., Ltd., Japan).
Furthermore, in the film formation using an electron beam deposition, it is generally known that radiation (typically X ray) is generated from a metal molten by irradiation with an electron beam. Thus, the present inventors suspected that abnormal characteristics of the thin film transistor (i.e., the threshold voltage thereof was shifted to the minus side) could be observed as a result of the generation of positive charges or the generation of interface state density caused on the gate insulating film or the like of the thin film transistor by the radiation caused at the time of the electron beam deposition.